'\" te
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.\" Copyright 2016 Joyent, Inc.
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.TH THREADS 7 "Mar 27, 2016"
.SH NAME
threads, pthreads \- POSIX pthreads, c11, and illumos threads concepts
.SH SYNOPSIS
.SS "POSIX"
.nf
gcc -D_REENTRANT [ \fIflag\fR... ] \fIfile\fR... [ \fIlibrary\fR... ]
.fi

.LP
.nf
#include <pthread.h>
.fi

.SS "C11"
.nf
gcc -std=c11 -D_REENTRANT [ \fIflag\fR... ] \fIfile\fR... [ \fIlibrary\fR... ]
.fi

.LP
.nf
#include <threads.h>
.fi

.SS "illumos"
.nf
gcc -D_REENTRANT [ \fIflag\fR... ] \fIfile\fR... [ \fIlibrary\fR... ]
.fi

.LP
.nf
#include <sched.h>
.fi

.LP
.nf
#include <thread.h>
.fi

.SH DESCRIPTION
A thread is an independent source of execution within a process. Every process
is created with a single thread, which calls the
.B main
function. A process may have multiple threads, all of which are scheduled
independently by the system and may run concurrently. Threads within a process
all use the same address space and as a result can access all data in the
process; however, each thread is created with its own attributes and its own
stack. When a thread is created, it inherits the signal mask of the thread which
created it, but it has no pending signals.
.sp
.LP
All threads of execution have their own, independent life time, though it is
ultimately bounded by the life time of the process. If the process terminates
for any reason, whether due to a call to \fBexit\fR(3C), the receipt of a fatal
signal, or some other reason, then all threads within the process are
terminated. Threads may themselves exit and status information of them may be
obtained, for more information, see the \fBpthread_detach\fR(3C),
\fBpthread_join\fR(3C), and \fBpthread_exit\fR(3C) functions, and their
equivalents as described in the tables later on in the manual.
.sp
.LP
Most hardware platforms do not have any special synchronization for data objects
which may be accessed concurrently from multiple threads of execution. To avoid
such problems, programs may use atomic operations (see \fBatomic_ops\fR(3C)) and
locking primitives, such as mutexes, readers/writer locks, condition variables,
and semaphores. Note, that depending on the hardware platform, memory
synchronization may be necessary, for more information, see \fBmembar_ops\fR(3C).
.LP
POSIX, C11, and illumos threads each have their own implementation within
\fBlibc\fR(3LIB). All implementations are interoperable, their functionality
similar, and can be used within the same application. Only POSIX threads are
guaranteed to be fully portable to other POSIX-compliant environments. C11
threads are an optional part of ISO C11 and may not exist on every ISO C11
platform. POSIX, C11, and illumos threads require different source and include
files. See \fBSYNOPSIS\fR.
.SS "Similarities"
Most of the POSIX and illumos threading functions have counterparts with each
other. POSIX function names, with the exception of the semaphore names, have a
"\fBpthread\fR" prefix. Function names for similar POSIX and illumos functions
have similar endings. Typically, similar POSIX and illumos functions have the
same number and use of arguments.
.SS "Differences"
POSIX pthreads and illumos threads differ in the following ways:
.RS +4
.TP
.ie t \(bu
.el o
POSIX threads are more portable.
.RE
.RS +4
.TP
.ie t \(bu
.el o
POSIX threads establish characteristics  for each thread according to
configurable attribute objects.
.RE
.RS +4
.TP
.ie t \(bu
.el o
POSIX pthreads implement thread cancellation.
.RE
.RS +4
.TP
.ie t \(bu
.el o
POSIX pthreads enforce scheduling algorithms.
.RE
.RS +4
.TP
.ie t \(bu
.el o
POSIX pthreads allow for clean-up handlers for \fBfork\fR(2) calls.
.RE
.RS +4
.TP
.ie t \(bu
.el o
illumos threads can be suspended and continued.
.RE
.RS +4
.TP
.ie t \(bu
.el o
illumos threads implement daemon threads, for whose demise the process does not
wait.
.RE
.SS "Comparison to C11 Threads"
C11 threads are not as functional as either POSIX or illumos threads. C11
threads only support intra-process locking and do not have any form of
readers/writer locking or semaphores. In general, POSIX threads will be more
portable than C11 threads, all POSIX-compliant systems support pthreads;
however, not all C environments support C11 Threads.
.sp
.LP
In addition to lacking other common synchronization primitives, the ISO/IEC
standard for C11 threads does not have rich error semantics. In an effort to not
extend the set of error numbers standardized in ISO/IEC C11, none of the
routines set errno and instead multiple distinguishable errors, aside from the
equivalent to ENOMEM and EBUSY, are all squashed into one. As such, users of the
platform are encouraged to use POSIX threads, unless a portability concern
dictates otherwise.

.SH FUNCTION COMPARISON
The following table compares the POSIX pthreads, C11 threads, and illumos
threads functions.  When a comparable interface is not available either in POSIX
pthreads, C11 threads  or illumos threads, a hyphen (\fB-\fR) appears in the
column.
.SS "Functions Related to Creation"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_create()\fR	\fBthr_create()\fR	\fBthrd_create()\fR
\fBpthread_attr_init()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_setdetachstate()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_getdetachstate()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_setinheritsched()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_getinheritsched()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_setschedparam()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_getschedparam()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_setschedpolicy()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_getschedpolicy()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_setscope()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_getscope()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_setstackaddr()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_getstackaddr()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_setstacksize()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_getstacksize()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_getguardsize()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_setguardsize()\fR	\fB-\fR	\fB-\fR
\fBpthread_attr_destroy()\fR	\fB-\fR	\fB-\fR
\fB-\fR	\fBthr_min_stack()\fR	\fB-\fR
.TE

.SS "Functions Related to Exit"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_exit()\fR	\fBthr_exit()\fR	\fBthrd_exit()\fR
\fBpthread_join()\fR	\fBthr_join()\fR	\fBthrd_join()\fR
\fBpthread_detach()\fR	\fB-\fR	\fBthrd_detach()\fR
.TE

.SS "Functions Related to Thread Specific Data"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_key_create()\fR	\fBthr_keycreate()\fR	\fBtss_create()\fR
\fBpthread_setspecific()\fR	\fBthr_setspecific()\fR	\fBtss_set()\fR
\fBpthread_getspecific()\fR	\fBthr_getspecific()\fR	\fBtss_get()\fR
\fBpthread_key_delete()\fR	\fB-\fR	\fBtss_delete()\fR
.TE

.SS "Functions Related to Signals"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_sigmask()\fR	\fBthr_sigsetmask()\fR	\fB-\fR
\fBpthread_kill()\fR	\fBthr_kill()\fR	\fB-\fR
.TE

.SS "Functions Related to IDs"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBc11\fR
\fBpthread_self()\fR	\fBthr_self()\fR	\fBthrd_current()\fR
\fBpthread_equal()\fR	\fB-\fR	\fBthrd_equal()\fR
\fB-\fR	\fBthr_main()\fR	\fB-\fR
.TE

.SS "Functions Related to Scheduling"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fB-\fR	\fBthr_yield()\fR	\fBthrd_yield()\fR
\fB-\fR	\fBthr_suspend()\fR	\fB-\fR
\fB-\fR	\fBthr_continue()\fR	\fB-\fR
\fBpthread_setconcurrency()\fR	\fBthr_setconcurrency()\fR	\fB-\fR
\fBpthread_getconcurrency()\fR	\fBthr_getconcurrency()\fR	\fB-\fR
\fBpthread_setschedparam()\fR	\fBthr_setprio()\fR	\fB-\fR
\fBpthread_setschedprio()\fR	\fBthr_setprio()\fR	\fB-\fR
\fBpthread_getschedparam()\fR	\fBthr_getprio()\fR	\fB-\fR
.TE

.SS "Functions Related to Cancellation"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_cancel()\fR	\fB-\fR	\fB-\fR
\fBpthread_setcancelstate()\fR	\fB-\fR	\fB-\fR
\fBpthread_setcanceltype()\fR	\fB-\fR	\fB-\fR
\fBpthread_testcancel()\fR	\fB-\fR	\fB-\fR
\fBpthread_cleanup_pop()\fR	\fB-\fR	\fB-\fR
\fBpthread_cleanup_push()\fR	\fB-\fR	\fB-\fR
.TE

.SS "Functions Related to Mutexes"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBc11\fR
\fBpthread_mutex_init()\fR	\fBmutex_init()\fR	\fBmtx_init()\fR
\fBpthread_mutexattr_init()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_setpshared()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_getpshared()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_setprotocol()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_getprotocol()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_setprioceiling()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_getprioceiling()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_settype()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_gettype()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_setrobust()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_getrobust()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutexattr_destroy()\fR	\fB-\fR	\fBmtx_destroy()\fR
\fBpthread_mutex_setprioceiling()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutex_getprioceiling()\fR	\fB-\fR	\fB-\fR
\fBpthread_mutex_lock()\fR	\fBmutex_lock()\fR	\fBmtx_lock()\fR
\fBpthread_mutex_timedlock()\fR	\fB-\fR	\fBmtx_timedlock()\fR
\fBpthread_mutex_trylock()\fR	\fBmutex_trylock()\fR	\fBmtx_trylock()\fR
\fBpthread_mutex_unlock()\fR	\fBmutex_unlock()\fR	\fBmtx_unlock()\fR
\fBpthread_mutex_destroy()\fR	\fBmutex_destroy()\fR	\fBmtx_destroy()\fR
.TE

.SS "Functions Related to Condition Variables"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_cond_init()\fR	\fBcond_init()\fR	\fBcnd_init()\fR
\fBpthread_condattr_init()\fR	\fB-\fR	\fB-\fR
\fBpthread_condattr_setpshared()\fR	\fB-\fR	\fB-\fR
\fBpthread_condattr_getpshared()\fR	\fB-\fR	\fB-\fR
\fBpthread_condattr_destroy()\fR	\fB-\fR	\fB-\fR
\fBpthread_cond_wait()\fR	\fBcond_wait()\fR	\fBcnd_wait()\fR
\fBpthread_cond_timedwait()\fR	\fBcond_timedwait()\fR	\fBcond_timedwait()\fR
\fBpthread_cond_signal()\fR	\fBcond_signal()\fR	\fBcnd_signal()\fR
\fBpthread_cond_broadcast()\fR	\fBcond_broadcast()\fR	\fBcnd_broadcast()\fR
\fBpthread_cond_destroy()\fR	\fBcond_destroy()\fR	\fBcnd_destroy()\fR
.TE

.SS "Functions Related to Reader/Writer Locking"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_rwlock_init()\fR	\fBrwlock_init()\fR	\fB-\fR
\fBpthread_rwlock_rdlock()\fR	\fBrw_rdlock()\fR	\fB-\fR
\fBpthread_rwlock_tryrdlock()\fR	\fBrw_tryrdlock()\fR	\fB-\fR
\fBpthread_rwlock_wrlock()\fR	\fBrw_wrlock()\fR	\fB-\fR
\fBpthread_rwlock_trywrlock()\fR	\fBrw_trywrlock()\fR	\fB-\fR
\fBpthread_rwlock_unlock()\fR	\fBrw_unlock()\fR	\fB-\fR
\fBpthread_rwlock_destroy()\fR	\fBrwlock_destroy()\fR	\fB-\fR
\fBpthread_rwlockattr_init()\fR	\fB-\fR	\fB-\fR
\fBpthread_rwlockattr_destroy()\fR	\fB-\fR	\fB-\fR
\fBpthread_rwlockattr_getpshared()\fR	\fB-\fR	\fB-\fR
\fBpthread_rwlockattr_setpshared()\fR	\fB-\fR	\fB-\fR
.TE

.SS "Functions Related to Semaphores"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBsem_init()\fR	\fBsema_init()\fR	\fB-\fR
\fBsem_open()\fR	\fB-\fR	\fB-\fR
\fBsem_close()\fR	\fB-\fR	\fB-\fR
\fBsem_wait()\fR	\fBsema_wait()\fR	\fB-\fR
\fBsem_trywait()\fR	\fBsema_trywait()\fR	\fB-\fR
\fBsem_post()\fR	\fBsema_post()\fR	\fB-\fR
\fBsem_getvalue()\fR	\fB-\fR	\fB-\fR
\fBsem_unlink()\fR	\fB-\fR	\fB-\fR
\fBsem_destroy()\fR	\fBsema_destroy()\fR	\fB-\fR
.TE

.SS "Functions Related to fork(\|) Clean Up"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_atfork()\fR	\fB-\fR	\fB-\fR
.TE

.SS "Functions Related to Limits"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fBpthread_once()\fR	\fB-\fR	\fBcall_once()\fR
.TE

.SS "Functions Related to Debugging"

.TS
l l l
l l l .
\fBPOSIX\fR	\fBillumos\fR	\fBC11\fR
\fB-\fR	\fBthr_stksegment()\fR	\fB-\fR
.TE

.SH LOCKING
.SS "Synchronization"
Multithreaded behavior is asynchronous, and therefore,  optimized for
concurrent and parallel processing. As threads, always from within the same
process and  sometimes from multiple processes, share global data with each
other, they are not guaranteed exclusive access to the shared data at any point
in time. Securing mutually exclusive access to shared data requires
synchronization among the threads. Both POSIX and illumos implement four
synchronization mechanisms: mutexes, condition variables, reader/writer locking
(\fIoptimized frequent-read occasional-write mutex\fR), and semaphores, where as
C11 threads only implement two mechanisms: mutexes and condition variables.
.sp
.LP
Synchronizing multiple threads diminishes their concurrency. The coarser the
grain of synchronization, that is, the larger the block of code that is locked,
the lesser the concurrency.
.SS "MT \fBfork()\fR"
If a threads program calls \fBfork\fR(2), it implicitly calls \fBfork1\fR(2),
which replicates only the calling thread. Should there be any outstanding
mutexes throughout the process, the application should call
\fBpthread_atfork\fR(3C) to wait for and acquire those mutexes prior to calling
\fBfork()\fR.
.SH SCHEDULING
.SS "POSIX Threads"
illumos supports the following three POSIX scheduling policies:
.sp
.ne 2
.na
\fB\fBSCHED_OTHER\fR\fR
.ad
.RS 15n
Traditional Timesharing scheduling policy. It is based on the timesharing (TS)
scheduling class.
.RE

.sp
.ne 2
.na
\fB\fBSCHED_FIFO\fR\fR
.ad
.RS 15n
First-In-First-Out scheduling policy. Threads scheduled to this policy, if not
preempted by a higher priority, will proceed until completion. Such threads are
in real-time (RT) scheduling class. The calling process must have a effective
user \fBID\fR of \fB0\fR.
.RE

.sp
.ne 2
.na
\fB\fBSCHED_RR\fR\fR
.ad
.RS 15n
Round-Robin scheduling policy. Threads scheduled to this policy, if not
preempted by a higher priority, will execute for a time period determined by
the system. Such threads are in real-time (RT) scheduling class and the calling
process must have a effective user \fBID\fR of \fB0\fR.
.RE

.sp
.LP
In addition to the POSIX-specified scheduling policies above, illumos also
supports these scheduling policies:
.sp
.ne 2
.na
\fB\fBSCHED_IA\fR\fR
.ad
.RS 13n
Threads are scheduled according to the Inter-Active Class (IA) policy as
described in \fBpriocntl\fR(2).
.RE

.sp
.ne 2
.na
\fB\fBSCHED_FSS\fR\fR
.ad
.RS 13n
Threads are scheduled according to the Fair-Share Class (FSS) policy as
described in \fBpriocntl\fR(2).
.RE

.sp
.ne 2
.na
\fB\fBSCHED_FX\fR\fR
.ad
.RS 13n
Threads are scheduled according to the Fixed-Priority Class (FX) policy as
described in \fBpriocntl\fR(2).
.RE

.SS "illumos Threads"
Only scheduling policy supported is \fBSCHED_OTHER\fR, which is timesharing,
based on the \fBTS\fR scheduling class.
.SH ERRORS
In a multithreaded application, \fBEINTR\fR can be returned from blocking
system calls when another thread calls \fBforkall\fR(2).
.SH USAGE
.SS "\fB-mt\fR compiler option"
The \fB-mt\fR compiler option compiles and links for multithreaded code. It
compiles source files with \(mi\fBD_REENTRANT\fR and augments the set of
support libraries properly.
.sp
.LP
Users of other compilers such as gcc and clang should manually set
\(mi\fBD_REENTRANT\fR on the compilation line. There are no other libraries or
flags necessary.
.SH ATTRIBUTES
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
MT-Level	MT-Safe, Fork 1-Safe
.TE

.SH SEE ALSO
.BR crle (1),
.BR fork (2),
.BR priocntl (2),
.BR pthread_atfork (3C),
.BR pthread_create (3C),
.BR libpthread (3LIB),
.BR librt (3LIB),
.BR libthread (3LIB),
.BR attributes (7),
.BR standards (7)
.sp
.LP
\fILinker and Libraries Guide\fR
